This invention relates to the field of reducing NO.sub.X emissions of combustors using steam injection.
The use of petrochemical off-gas blends to generate power at refineries would be advantageous but for the hydrogen percentage and how it affects flashback and NO.sub.X emissions. Petrochemical off-gas blends have hydrogen concentrations of 30-40% by volume, which is significantly higher than that of natural gas.
High hydrogen containing fuels increase the opportunity for detrimental flashback. Hydrogen has a flame speed that is an order of magnitude higher than natural gas. As such, a hydrogen flame has an increased potential to flashback, or travel upstream into the premixing region. Extended operation under these conditions will cause a significant increase in the NO.sub.X emissions, and damage to hardware may occur.
Flashback may be avoided, but the expense of generating increased NO.sub.X emissions, by increasing the percentage of fuel to the diffusion flame pilot of the combustor relative to the total amount of fuel sent to the combustor. However, the higher fuel percentage in the diffusion flame pilot nozzle, the higher the NO.sub.X emissions.
Further, just the use of high hydrogen fuel increases the potential for increased NO.sub.X generation. The generation of NO.sub.X is increased with higher combustion temperatures. High hydrogen fuel has a higher adiabatic flame temperature than that of natural gas. Burning the high hydrogen fuel results in higher combustion temperatures which correlates to higher NO.sub.X.
The prior art discloses the beneficial results of injecting steam and/or water into a combustor. The addition of steam or water into the combustor reduces the amount of NO.sub.X produced at least in part by reducing flame temperature. Further, steam/water injection also reduces NO.sub.2 in the emission, resulting in elimination of yellow-tinted emissions. Steam can also be added to the combustor when it is not running at full capacity to keep NO.sub.X emissions below predetermined limits. This would be beneficial when combusting high hydrogen fuels.
The prior art discloses adding steam and/or water to the combustor such that it is distributed throughout the combustion zone of the combustor, thus generally affecting combustion. For example, U.S. Pat. No. 4,089,639 discloses premixing water vapor with fuel prior to entering the combustor. In another example, U.S. Pat. No. 5,404,711 discloses premixing water with the air stream prior to combustion.
However, the injection of steam and/or water into the combustor results in undesirably higher plant heat rates. The generation of the steam takes energy out of the plant, and increases the heat rate. The addition of steam reduces the flame temperature and, typically, combustor efficiency. Therefore, a need exists for a combustion system and method that has reduced NO.sub.X emissions and uses less steam, resulting in beneficially decreased plant heat rates.